A combustor comprising a combustion chamber having an axial direction and a radial direction is, e.g., described in U.S. Pat. No. 6,532,726 B2. The combustor described therein consists of a burner with a burner head portion to which a radial inflow swirler is attached, a combustion pre-chamber and a combustion main chamber following the pre-chamber in an axial direction of the combustor. The main chamber has a diameter larger than that of the pre-chamber. The swirler defines a number of straight air passages between swirler vanes. Each air passage extends along a straight line which is perpendicular to the axial direction of the combustor. Moreover, this straight line has an inclination angle relative to the radial direction of the combustor so that the in-streaming air has a tangential component with respect to a circle around the combustor's axial direction. The direction of air streaming through the swirler into the pre-chamber has therefore a radial and a tangential component with respect to said circle. The main fuel for the combustion process is introduced into the air stream streaming through the air passages. The burner is a so-called premix burner in which a fuel and air are mixed before the mixture is burned.
The concept of pre-mixing fuel and air is generally used in modern gas turbine engines for reducing undesired pollutants in the exhaust gas of the combustion. There are two main measures by which a reduction of pollutants is achievable. The first is to use a lean stoichiometry, e.g. a fuel/air mixture with a low fuel fraction. The relatively small fraction of fuel leads to a combustion flame with a low temperature and thus to a low rate of nitrous oxide formation. The second measure is to provide a thorough mixing of fuel and air before the combustion takes place. The better the mixing is, the more uniformly distributed the fuel in the combustion zone. This helps to prevent hot spots in the combustion zone which could arise from relative local maxima in the fuel/air mixing ratio, i.e. zones with high fuel/air mixing ratio compared to the average fuel/air mixing ratio in the combustor.